Interpretive Summary: Polyhydroxyalkanoates (PHAs) are plastic-like polymeric materials that are made by many bacterial species from renewable resources. Their properties, including strength and rigidity, have historically been controlled by the incorporation of different components into the long-chain molecules. Sophorolipids (SLs) are surface-active molecules that have been established to gather at interfaces (e.g. the air/water interface) and affect the surface tension of liquids while also possessing antibacterial and immune-modulating properties. PHAs have been documented to be effective replacements for petroleum-based plastics in many applications however; poly-3-hydroxybutyrate (PHB), which is the most prevalent of all PHAs, is brittle and therefore not ideal for applications where structural integrity is paramount. By utilizing SLs as blending materials for PHB and other PHA molecules, we demonstrated that the physical properties of PHA films can be controlled through the formation of SL-induced dimples on the surface of the films and increased porosity in the film matrices. By utilizing SL as a blending material with rigid PHA molecules, we were able to lower the rigidity of the PHA films and impart the capability for slow-release antibacterial SL. This discovery lends itself favorably to biomedical applications in such areas as tissue scaffolding where film surface roughness and porosity are important with the added benefit of SL to help guard against infection and an adverse immune response.

Technical Abstract:
Sophorolipids (SL; microbial glycolipids) were used as additives in solvent-cast short-chain polyhydroxyalkanoate (sc-PHA) films to enhance surface roughness and porosity. Poly-3-hydroxybutyrate (PHB), poly-(6%)-3-hydroxybutyrate-co-(94%)-3-hydroxyvalerate (PHB/V), and poly-(90%)-3-hydroxybutyrate-co-(10%)-3-hydroxyhexanoate (PHB/HHx) films were evaluated with up to 43 wt% of SL. Sophorolipid addition caused surface dimples with maximum diameters of 131.8 micrometers (PHB), 25.2 micrometers (PHB/V), and 102.8 micrometers (PHB/HHx). A rise in the size and number of pores in the polymer matrix also occurred in PHB and PHB/V films. Surface roughness and film porosity were visualized by scanning electron microscopy and quantitated using confocal microscopy by correlating the surface area (A’) to the scanned area (A) of the films. The phenotypic alterations of the films caused a gradual decline in tensile strength and modulus and increased the elongation to break. Reductions in the enthalpies of fusion (Delta Hf) in both the PHB (41% reduction) and PHB/HHx (36% reduction) films indicated diminished crystallinity as SL concentrations increased. Over the same SL concentrations the Tan Delta maxima shifted from 4 degrees C to 30 degrees C and from 2 degrees C to 20 degrees C in these respective films. These results provide a potential new use for SL and a novel means by which sc-PHA properties can be controlled for new/improved applications.